The present invention relates to a polyolefin laminate paper used for electrical insulation which is formed by bonding together fiber paper layers and a molten and extruded polyolefin layer. The invention relates more specifically to such a polyolefin laminate paper in which the amount of thickness increase due to swelling is reduced, but with no deterioration of the electrical and mechanical characteristics thereof caused when the polyolefin layer is swelled by insulating oil. The invention also relates to an electric power supply cable incorporating the laminate paper.
Polyolefin laminate paper has an excellent dielectric characteristic and high dielectric strength. Therefore, it is often used as an insulating material for electrical equipment such as ultra- or ultra-super high voltage cables. However, because of various difficulties, polyolefin laminate paper has not found wide application as an insulating material for more common types of electrical equipment. The most significant difficulty is that a polyolefin film layer which forms a part of the laminate paper is swelled by the insulating oil in the electrical equipment, for instance, a hydrocarbon insulating oil such as mineral oil, alkyl benzene or alkyl naphthalene oil, as a result of which the thickness of the film layer is increased.
The increase of thickness results in the following two problems:
Firstly, the increase in thickness of the polyolefin film layer lowers the fluidity of the insulating oil in the insulating layer which is required for compensating for the expansion or contraction of the insulating oil caused as the temperature of the electrical equipment changes.
In conventional laminated kraft paper insulated electrical equipment, the kraft papers are porous, and therefore the insulating oil can flow in the widthwise direction of the kraft papers. On the other hand, in a polyolefin laminate paper, due to the presence of a plastic film layer, it is difficult for the insulating oil to flow in the widthwise direction of the laminate paper. Accordingly, the insulating oil is forced to flow along the surfaces of the laminate paper through the air gaps formed by irregularities on the surfaces of the kraft papers or the air gaps among the fibers of the kraft papers, or it is forced to flow through the gap between the laminate papers. Thus, even before the thickness of the film layer is increased, the fluidity of the insulating oil is essentially low. This difficulty may be overcome to some degree by employing kraft paper whose impermeability is relatively low. However, when the film layer increases in thickness upon being swelled in the insulating oil, then the kraft paper layers are compressed, as a result of which the air gaps are reduced and the fluidity of the insulating oil is lowered. That is, it is impossible for the insulating oil to flow in the electrical equipment as required.
The second problem occurs with compression of the kraft paper layers. Compression of the kraft paper layers increases the surface pressure between adjacent tapes, thus making it difficult for the tapes to slide on each other. For instance, when a cable is bent, the tapes can be creased or possibly cracked. Thus, the quality of the cable is lowered by the compressed kraft paper layers.
These phenomena become significant with an increase of thickness of the film layer or the percentage of the thickness of the polyolefin film layer with respect to the entire thickness of the polyolefin laminate paper. Accordingly, it is necessary to reduce the percentage of the thickness of the polyolefin film layer with respect to the entire thickness of the polyolefin laminate paper as the entire thickness of the polyolefin laminate paper increases.
The swelling of the polyolefin laminate paper for electrical insulation may be suppressed by various methods. A first method is a so-called "annealing method" in which, for instance, in the case of a cable, after taping, a heat treatment is carried out in a vacuum at a temperature lower than the melting point of polyolefin for a predetermined period of time to increase the degree of crystallinity of the polyolefin film, thereby to suppress the swelling in the insulating oil. A second method is a so-called "humidity adjusting method" in which cellulose fiber paper layers are caused to absorb moisture to expand in advance, and the decrease in thickness of the fiber paper layers which is caused by drying after the taping operation is utilized to suppress the increase in thickness of the polyolefin film layer which is caused by the swelling. A third method is a so-called "after-embossing method" in which a laminate paper, formed by combining fiber papers and a polyolefin film, is embossed so that irregularities are mechanically formed thereon. Deformation of the irregularities is utilized to absorb the increase in thickness of the film layer which is caused when it is swelled in the insulating oil.
As object of the invention is to provide a novel polyolefin laminate paper in which the increase in thickness which is caused as the laminate paper is swelled in insulating oil is reduced without deteriorating the electrical and mechanical characteristics thereof. That is, the invention relates to a polyolefin laminate paper which is formed by combining at least one fiber paper layer, which has been embossed so that irregularities are formed thereon, with a polyolefin film layer, which is formed by a melting and extruding method, in order that the increase in thickness due to the swelling thereof is absorbed by flattening the irregularities of the fiber paper layer and polyolefin film layer thus combined.
In a conventional embossing method, a laminate paper obtained by combining fiber papers and a plastic film in advance is embossed later so that irregularities are mechanically formed thereon. Therefore, the thusproduced laminate paper often suffers from difficulties that it is locally damaged, and therefore its dielectric strength is lowered and the bonding strength of the fiber paper and the plastic film decreased.
Furthermore, in this after-embossing method, it is difficult to provide irregularities in the laminate paper used as an insulating layer which are relatively small and sufficient to absorb the increase in thickness of the laminate paper which is caused when it is swelled in the oil. This is due to the following reasons: The Young's modulus in the widthwise direction of the laminate paper which is obtained by combining fiber papers and a plastic film is three or four times as large as that in the widthwise direction of the fiber papers only. Accordingly, an embossing pressure larger than the ordinary embossing pressure should be employed to emboss the laminate paper. However, if the embossing pressure is excessively large, then the laminate paper is greatly damaged, and its mechanical and electrical characteristics deteriorated. Furthermore, irregularities imparted to the laminate paper are larger in size than those which are just sufficient to absorb the increase in thickness of the laminate paper which is caused as it is swelled in the oil. Thus, the embossed laminate paper is not suitable as an electrical insulating material. Still further, the embossing member tends to wear quickly, and hence it is difficult to uniformly emboss laminate papers. On the other hand, if the embossing pressure is low, it is difficult to impart irregularities to the surfaces of the laminate paper. Accordingly, it is difficult to maintain a suitable operating range in which minute irregularities are continuously imparted to a laminate paper to the extent that the mechanical characteristics and the dielectric strength are not lowered and the increase in thickness due to the swelling is suppressed. This makes it difficult to apply the embossed laminate paper in many common electrical equipment applications.